1. Field of the Invention
This invention relates to the removal of hydrogen sulfide from gas streams by contacting the gas with an aqueous washing solution, and more particularly to an improved method for the removal of hydrogen sulfide from gas streams in which the undesirable formation of water-soluble surlfur-containing salts and the undesirable chemical consumption of the constituents of the washing solution are substantially reduced.
2. Description of the Prior Art
With the increasing concern over the pollution of the atmosphere, great demands have been made on industry to produce pollution-free products in a pollution-free manner. One area of particular concern has been the release of sulfur and its compounds into the atmosphere as a result of the refining of petroleum, the sweetening of sour natural gas, the processing of ores, the destructive distillation of coal and/or oil shale, the gasification or liquefaction of coal, the use of geothermal fluids to generate electricity, or other processes which produce hydrogen sulfide-containing gases.
Various processes have been employed to remove hydrogen sulfide from gas streams. The Stretford process and the Takahax process employ aqueous alkaline washing solutions to absorb the hydrogen sulfide from the gas stream and to convert the absorbed hydrogen sulfide primarily to sulfur. The solution is regenerated by flowing air through the solution. The product sulfur is separated from the regenerated solution before recycling the solution to the absorber. The washing solution employed in the Stretford process contains a water-soluble salt of 9,10-anthraquinone disulfonic acid (ADA) and a water-soluble vanadate or other multivalent metal compound. The washing solution employed in the Takahax process contains a water-soluble naphthoquinone sulfonate (NQS) alone or in combination with a multivalent metal compound. While these processes are successful in substantially reducing the hydrogen sulfide content of the gas streams being treated, several problems exist which tend to increase their operating expenses. One such problem involves the chemical consumption of the washing solution constituents, such as ADA, NQS and/or the alkaline constituents of the solution. The cost of chemicals required to replenish the supply of these consumable constituents adds significantly to the cost of the process. Another of the problems involves the undesirable conversion of the absorbed hydrogen sulfide and/or the product sulfur to water-soluble sulfur-containing salts, such as sulfates, thiosulfates and polythionates which accumulate in the washing solution. These accumulated salts tend to (1) increase the corrosivity of the washing solution, (2) cause precipitation of the desired constituents of the washing solution and (3) increase the rate of chemical consumption of the alkaline constituents of the washing solution.
In commercial plants employing these processes, attempts have been made to mitigate these problems either by periodic replacement of the washing solution or by intermittently or continuously bleeding a slipstream of the contaminated washing solution while adding make-up chemicals to replenish the circulating solution. Both procedures are unsatisfactory due to the expense of the make-up chemicals and the difficulty involved in disposal of the contaminated solution.
It has been proposed to modify the washing solution of the Stretford process in order to reduce the chemical consumption of ADA, such as by adding to the conventional Stretford solution a thiosulfate ion precursor (U.S. Pat. No. 3,972,989 to Fenton et al.), a thiocyanate ion precursor (U.S. Pat. No. 4,017,594 to Fenton et al.), or a water-soluble inorganic fluoride, borate or phosphate complexing agent (U.S. Pat. No. 4,060,594 to Fenton et al.). It has also been proposed to modify the process steps of the Stretford process in order to reduce the undesirable formation of water-soluble sulfur-containing salts, such as by the use of multistage absorbers (U.S. Pat. No. 4,125,597 to Fleck) or an oxygen-stripping device (U.S. Pat. No. 3,642,448 to Beavon) to remove oxygen from the washing solution before it is contacted with the hydrogen sulfide-containing gas stream. It has also been proposed to selectively separate the undesirable water-soluble sulfur-containing salts from the washing solution in order to reduce the contaminant concentration, such as by the addition of a water-soluble aldehyde to the solution to convert the thiosulfate salts to sulfate salts which are then separated from the solution by cooling the solution to selectively precipitate the sulfate salts. (U.S. Pat. No. 4,083,945 to Fenton et al.)
While the aforementioned modifications have been somewhat successful in reducing one or more of the problems associated with the Stretford process, these modifications have not been entirely satisfactory in all instances. Thus a need exists for a hydrogen sulfide removal process which has all the beneficial features of the Stretford process but which substantially eliminates the aforementioned problems.
Accordingly, a primary object of this invention is to provide a method for removing hydrogen sulfide from gas streams.
Another object of this invention is to provide a hydrogen sulfide removal method in which the hydrogen sulfide content of the gas being treated is reduced to an acceptably low level at a reasonable cost.
Still another object of this invention is to provide a method for removing hydrogen sulfide from gas streams in which the absorbed hydrogen sulfide is converted substantially exclusively to sulfur and wherein the undesirable conversion of hydrogen sulfide and/or the product sulfur to water-soluble sulfur-containing salts is substantially reduced or eliminated.
Yet another object of this invention is to provide a method in which hydrogen sulfide is absorbed from a gas stream by a regenerable washing solution and in which the chemical consumption of the washing solution constituents is substantially reduced or eliminated.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawings.